Method and circuitry for implementing a differentially tuned varactor-inductor oscillator

ABSTRACT

The present invention generally relates to voltage-controlled oscillators. More specifically, the present invention relates to method and circuitry for implementing a differentially tuned varactor-inductor oscillator. In one exemplary embodiment, the present invention includes an LC tank circuit having a couple of terminals, a first and second capacitors, and a first and second varactors. The first and second varactors are connected in series forming a first and a second node. The first capacitor connects the first node and one terminal of the LC tank circuit. The second capacitor connects the second node and the other terminal of the LC tank circuit. A pair of differential input control signals is applied across the first and the second varactors, respectively, to tune the LC tank circuit thereby generating an oscillator output.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims the benefit of priority under 35 U.S.C. §119 from the provisional patent application, U.S. Patent ApplicationSer. No. 60/184,721, filed on Feb. 24, 2000, which is herebyincorporated by reference as if set forth in full in this document.

BACKGROUND OF THE INVENTION

The convergence of various high speed data communication technologies(e.g., Ethernet, fiber channel, IEEE firewire links) into the gigabitdomain has focused the efforts of integrated circuit designers ondeveloping high speed circuit techniques for processing broadbandsignals. A circuit block that is commonly found in these types ofcommunication applications is a voltage-controlled oscillator (VCO). Asthe main building block of phase-locked loops (PLLs), the VCO can befound, for example, in clock and data recovery circuits.

The VCO is typically an important element in determining the overallnoise performance of a PLL. Since the VCO is the part of the PLL whichproduces an ac output signal whose frequency is proportional to theinput control signal, external unwanted noise affecting the inputcontrol signal has an adverse impact on the performance of the PLL.Hence, it is desirable to provide a VCO whose input control signal isless susceptible to noise interference thereby improving the performanceof the PLL.

Furthermore, conventional VCOs are constructed to provide a single-endedoutput signal. FIG. 1 is a simplified circuit diagram showing aconventional design of a VCO. Under this conventional design, the VCOincludes an active circuit (or driver) 10 having a couple of terminalsA, B, an inductor-capacitor (LC) tank circuit 12 and a couple ofvariable capacitors (or varactors) 14, 16 connected in series. The LCtank circuit 12 and the varactors 14, 16 are connected in a parallelmanner across the terminals A, B. The input control signal V_(tune) isconnected to the node between the two varactors 14, 16. As FIG. 1 shows,the input control signal V_(tune) is single-ended.

Due to their single-ended nature, single-ended signals, however, aremore susceptible to noise interference. It is, therefore, desirable toprovide a VCO implemented in CMOS technology based on aninductor-capacitor (LC) oscillator structure that uses differentiallycontrolled varactors.

SUMMARY OF THE INVENTION

The present invention generally relates to voltage-controlledoscillators. More specifically, the present invention relates to methodand circuitry for implementing a differentially tuned varactor-inductoroscillator implemented using CMOS technology.

In one exemplary embodiment, the present invention includes an LC tankcircuit having a couple of terminals, a first and second capacitors, anda first and second varactors. The first and second varactors areconnected in series forming a first and a second node. The firstcapacitor connects the first node and one terminal of the LC tankcircuit. The second capacitor connects the second node and the otherterminal of the LC tank circuit. A pair of differential input controlsignals is applied across the first and the second varactors to tune theLC tank circuit thereby generating an oscillator output.

Accordingly, in one embodiment, the present invention provides avoltage-controlled oscillator including: an LC tank circuit, a pluralityof varactors each having a first and a second terminal; and a pluralityof capacitors respectively coupling said LC tank circuit to saidplurality of varactors; wherein, a pair of differential input controlsignals are applied across said first and second terminals of each ofsaid plurality of varactors to tune said LC tank circuit so as togenerate an oscillator output.

Accordingly, in another embodiment, the present invention provides amethod for implementing a voltage-controlled oscillator including:connecting a first varactor and a second varactor in series therebyforming a first and a second node; coupling a first capacitor between anLC tank circuit and said first node; coupling a second capacitor betweensaid LC tank circuit and said second node; and applying a pair ofdifferential input control signals across said first varactor and saidsecond varactor respectively.

Reference to the remaining portions of the specification, including thedrawings and claims, will realize other features and advantages of thepresent invention. Further features and advantages of the presentinvention, as well as the structure and operation of various embodimentsof the present invention, are described in detail below with respect toaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified circuit diagram showing a conventional design ofa VCO;

FIG. 2 is a simplified circuit diagram of an exemplary embodiment of thepresent invention; and

FIG. 3 is a simplified circuit layout of the exemplary embodiment asshown in FIG. 2.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The present invention will now be described. FIG. 2 shows a simplifiedcircuit diagram of an exemplary embodiment of the present invention. Asshown in FIG. 2, the exemplary embodiment shows a VCO 18 having anactive circuit 20 having a couple of terminals X,Y, an LC tank circuit22, a first capacitor 24, a second capacitor 26, a first resistor 28, asecond resistor 30, a first varactor 32 and a second varactor 34.Further, the LC tank circuit 22 includes an inductor 38 and a thirdcapacitor 26. These various elements are connected as follows.

The LC tank circuit 22 is connected across the terminals X and Y of theactive circuit 20. The two resistors 28 and 30 are connected in series.The two varactors 32 and 34 are, similarly, connected in series. The tworesistors 28 and 30 in series and the two varactors 32 and 34 in seriesare then connected in a parallel manner forming a first node C and asecond node D. The first capacitor 24 is then used as a bridgeconnecting the first node C and the terminal X; likewise, the secondcapacitor 26 connects the second node D and the terminal Y. A pair ofdifferential input control signals V_(tune) and V_(tunen) are applied atthe nodes between the two resistors 28 and 30 and the two varactors 32and 34, respectively to achieve tuning of the LC tank circuit 22. FIG. 3is a simplified circuit layout of the exemplary embodiment as shown inFIG. 2 using CMOS technology.

The operation of the exemplary embodiment as shown in FIG. 2 isdescribed as follows. Referring back to FIG. 2, the two capacitors 24and 26 are coupled between the LC tank circuit 22 and the first andsecond node C and D respectively. By their functional nature, thecapacitors 24 and 26 block off any DC current into the LC tank circuit22 from the first and second node C and D. Hence, the two varactors 32and 34 are, in effect, AC coupled to the LC tank circuit 22.

Since no DC current flows into the LC tank circuit 22, thisconfiguration allows both terminals of the two varactors 32 and 34 to beconnected to any arbitrary DC voltage potentials. This is because thepair of differential input control signals V_(tune) and V_(tunen) arenot limited to a voltage range within the power supply. Instead, thepair of differential input control signals V_(tune) and V_(tunen) can beprovided at any arbitrary DC voltage potentials thereby allowing thevoltage across the varactors 32 and 34 to assume any arbitrary DCvoltage potential as well. As shown in FIG. 2, differential inputcontrol signals V_(tune) and V_(tunen) are applied to the varactors 32and 34 and the LC tank circuit 22 via AC coupling capacitors 24 and 26.The VCO 18 can thus be differentially controlled which results insignificantly improved noise performance.

In one embodiment, the two varactors 32 and 34 are implemented by thegate oxide of an MOS transistor structure inside a well region. Given ap-type silicon substrate, for example, the MOS varactors would beconstructed of a polysilicon-gate oxide-n-type silicon sandwich insidean n-well. The n-well which essentially acts as the bottom plate of thevaractor provides isolation from the substrate. Alternatively, the twovaractors 32 and 34 can also be implemented using junction varactors.

The AC coupling capacitors 24 and 26 each has a fixed capacitance andcan be implemented by a number of different capacitor structuresincluding an MOS capacitor, polysilicon-insulator-polysilicon, ormetal-insulator-metal. In a preferred embodiment, the AC couplingcapacitors 24 and 26 are implemented using the metal-insulator-metal(MIM) type of capacitor as MIM type of capacitors exhibit minimum seriesresistance. In addition, the MIM capacitor is much more linear ascompared to an MOS capacitor. A person of ordinary skill in the art willknow of other methods and ways to implement the AC coupling capacitors24 and 26 and the two varactors 32 and 34.

A significant advantage of the differentially controlled VCO of thepresent invention is that common mode noise is rejected from the LC tankcircuit 22, and therefore does not adversely affect the frequency of theVCO 18. This is particularly advantageous in applications such as PLLcircuits where analog circuits share the same substrate with noisydigital circuitry that are in close proximity.

Yet another advantage of the present invention is that the tuning rangeof the varactors 32 and 34 is expanded since larger differentialvoltages can be applied as compared to the conventional single-endedimplementations.

It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art and areto be included within the spirit and purview of this application andscope of the appended claims. All publications, patents, and patentapplications cited herein are hereby incorporated by reference for allpurposes in their entirety.

1. A voltage-controlled oscillator comprising: an LC tank circuit; aplurality of varactors each having a first and a second terminal; and aplurality of capacitors respectively coupling said LC tank circuit tosaid plurality of varactors; wherein a pair of differential inputcontrol signals are applied across said first and second terminals ofeach of said plurality of varactors to tune said LC tank circuit so asto generate an oscillator output. 2-12. (canceled)